Make sure the Healer isn't correcting the speedo...I forget which DIP switch it is...check the manual. (or do the following part before installing)
I hopped on one of the highways, we have mile markers every 2/10 mile. I pulled off to the side right at a '0' mile marker (eg, 210.0). I reset my trip meter to 0.0 and started riding for 20 miles. At 20.0 by the markers, I pulled to the side and looked at my tripmeter indicated.
Next, once you have the actual and indicated values, goto Speedohealer's site and use the Online Calculator to find your switch settings
I setup the SoH and rode the 20 miles again...bingo, right-on!
If you're a little off, just re-adjust the values.
Hope this helps!